1. Technical Field
The present invention relates to a fiber-optic gyroscope, and more particularly to a fiber-optic gyroscope which can correct angular velocity information.
2. Background Art
A mechanical gyroscope which employs a rotating system is conventionally used to control an attitude of, for example, airplanes and rockets, and in recent years a fiber-optic gyroscope which employs a loop optical fiber instead of the rotating system is also used.
FIG. 2 of the accompanying drawings shows the schematic construction of such a fiber-optic gyroscope. In this illustration, a drive circuit for a light source module 1 is not shown. The light source module 1 incorporates a laser diode which emits laser beams in two opposite directions (toward items 2a and 9 respectively in the illustration). A high birefringence polarization maintaining (PM) fiber extends to a directional coupler 2a from the light source module 1 and one light emitted from the laser diode of the light source module 1 is divided into two lights by the directional coupler 2a. One of them passes through a polarizer 3 and, is further divided into two lights by a second directional coupler 2b. One of these two lights enters a PM fiber loop 5 via a connection point 4 and propagates in the loop 5 counterclockwise whereas the other passes through an optical phase modulator 6 via another connection point 4, enters the PM fiber loop 5 and propagates in the loop 5 clockwise. The lights which have entered the PM fiber loop 5 respectively exit from the loop 5 at opposite ends of the loop 5 and proceed through the directional coupler 2b, the polarizer 3 and the directional coupler 2a respectively and these lights are received by a light receiver 7. Then, a signal processing is performed in a signal processing circuit 8 to obtain an angular velocity and an angle and to control the degree of modulation of the optical phase modulator 6. This phase modulator 6 is controlled with respect to its modulation degree by applying an alternating voltage to a PZT cylinder of the phase modulator 6. Generally, a PM fiber of 2-3 m (or may be more) is around the PZT cylinder to constitute the optical phase modulator 6.
Meantime, the other light emitted from the laser diode of the light source module 1 proceeds to the signal processing circuit 8 through the diode 9 and it is used to control the amount of the opposite light (tile one directed to the directional coupler 2a).
The fiber-optic gyroscope causes the light from the light source to propagate into the optical fiber, detects a phase difference .DELTA..theta. (Sagnac Effect) produced between the light propagating clockwise and counterclockwise in the loop, and obtains the angular velocity .OMEGA..
The relation between this phase difference .DELTA..theta. and the angular velocity .OMEGA. is expressed by the following formula: EQU .DELTA..theta.=(4.pi.RL/.lambda.c)*.OMEGA. (1)
Here, R represents a radius of the sensing loop 5, L represents the length of the optical fiber, .lambda. represents the wavelength of the light and c represents the light velocity. Among the constants of the formula (1), the wavelength .lambda. has the largest degree of fluctuation.
FIG. 3 of the accompanying drawings is a graph showing a light source temperature-wavelength characteristic and FIG. 4 of the accompanying drawings is a graph showing a current-wavelength characteristic.
In FIG. 3, the horizontal axis indicates the temperature and the vertical axis indicates the wavelength, and it can be appreciated that the wavelength increases as the temperature rises. In FIG. 4, on the other hand, the horizontal axis indicates the current and tile vertical axis indicates the wavelength, and it is appreciated that the wavelength decreases as the current increases. Therefore, since the relation between the phase difference .DELTA..theta. and the angular velocity .OMEGA. varies with the temperature and/or the current, conventional following methods are taken as countermeasures:
(1) To attach a Peltier element to the light source so as to maintain the light source module 1 at a constant temperature while driving the light source with a constant current circuit; and PA1 (2) To drive the light source with an APC (Auto-Power Control) circuit and monitor the temperature by a thermistor located near the light source. A correction value is obtained from a temperature detected by the thermistor, and multiplying the formula (1) by the correction value results in the corrected angular velocity information. PA1 (i) An applicable temperature range is not wide, i.e., the range which can maintain the temperature with the Peltier element is 0.degree.-40.degree. C.; PA1 (ii) Power consumption of the entire system which incorporates the fiber-optic gyroscope is large since the power used for the Peltier element is large; and PA1 (iii) System is expensive.
However, the above method (1) has following problems:
Method (2) is problematic in that since the current flowing in the light source module 1 fluctuates, a perfect correction cannot be expected.
Another type of optic-gyroscope is proposed in a certain Japanese Patent Application (Publication No. 3-92734, published Apr. 17, 1991). The invention disclosed in this publication is directed to a method of measuring a temperature using a fiber-optic sensor. The temperature as measured is used to correct various parameters of a fiber-optic sensor. Specifically, according to the teaching of this publication, the temperature around a light source is obtained from the relation between an intensity of light output from the light source and a current applied to drive the light source. Then, the temperature is used for the compensation of scale factor fluctuation. However, this technique has a following problem: A discrepancy between an optical axis of the light source placed in a light source module and an optical axis of the optical fiber appears as the temperature changes. This is an optical output change due to a mechanical structure. It is believed that the relation between the intensity of the light and the drive current is not enough to determine the temperature in the vicinity of the light source. Accordingly, a perfect scale factor compensation cannot be expected.